Update.
stripped and cleaned the carb for the 3rd time, nothing obvious. Took the reed block off, cleaned and replaced, nothing obvious again. Throttle cable was very slightly kinked, so changed out for a lined new one. All reassembled and now working fine. Then only thing I found was the very minor kink in the throttle cable, which I can not see as being the issue, but as it’s now working fine, going to stop worrying and just ride it.
thanks for your help